The present invention relates to novel, light-sensitive polycondensation products of aromatic diazonium salts, which products are suitable for the preparation of light-sensitive recording materials, particularly planographic printing plates, that can be developed with water.
It is known to prepare polycondensation products by reacting an aromatic diazonium salt, especially a diphenylamine-4-diazonium salt, with a second component carrying active methylol groups or esters or ethers thereof, the reactive groups being introduced in the simplest case by reacting the second component with formaldehyde. These condensation products give high-grade, radiation-sensitive materials, particularly printing plates, and are widely utilized industrially. They are described in U.S. Pat. No. 3,849,392.
All printing plates used in industry that are based on these condensation products are developed with solutions that are predominantly aqueous. But these solutions always require quite considerable quantities of other additives, such as alkaline or acidic buffer salts, water-miscible organic solvents and wetting agents. Such additives become necessary in particular if the light-sensitive layer also contains polymeric binders in addition to the diazonium salt polycondensation products. Plates of this type are described, for example, in European Patent Application No. 48,876 and 71,881.
For reasons of easy processability and, in particular, for reducing pollution of the environment, the ability to develop planographic printing plates solely with water would be desirable.
Although it is known from German Auslegeschrift No. 1,214,086 that negative-working planographic printing plates based on diphenylamine4-diazonium salt/formaldehyde condensation products can be obtained by developing with pure water, these plates give only a relatively short print run and have a relatively low light sensitivity. Moreover, the ink acceptance of these printing forms is inadequate in most cases, so that additional treatment of the developed plate with an oleophilic coating is necessary.
Finally, German Offenlegungsschrift No. 2,041,395 has disclosed a light-sensitive mixture for the preparation of screen printing forms, which mixture contains a water-soluble diazonium salt cocondensation product of the type described above, which is present as the salt of a lower aliphatic monosulfonic acid. These condensation products have a higher light sensitivity than the abovementioned formaldehyde condensation products. They are always used in combination with a hydrophilic polymer, but have been described exclusively with reference to the preparation of screen printing stencils and can be developed with pure water. This patent publication does not mention also using the condensation products for planographic printing; the disclosed mixtures would, in fact, be unsuitable for this purpose because of their high hydrophilic binder content.
It has also been found that these diazonium salt condensation products tend, in precipitation with methanesulfonic acid or salts thereof, to carry down a relatively large quantity of impurities. The precipitation products must therefore be laboriously purified if products of reproducible composition and activity are to be obtained. Other condensation products of this type must, after precipitation, be stirred for a prolonged period at an elevated temperature, until they have been converted into a form which can be filtered.